OK, you can manually convert Thunderbird email files to Outlook Express or Outlook, by following the procedures here:
http://www.ehow.com/how_5879683_conv...k-express.html
http://www.ehow.com/how_8670030_conv...d-outlook.html
The only caveat seems to be that you need to have Firefox (which is what Thunderbird is part of) installed. You can uninstall it again afterwards, of course. There are various companises that market inexpensive conversion utiltities also.
Juno is an email provider, like gmail, rather than an email application. You can configure both Outlook Express and Outlook to work with such providers.
No email propgram I am aware of generates .frm or .mbs files. There other programs that do, though.